Under the wireless local area network (WLAN) IEEE Standard 802.11, wireless stations (STA) and access points (AP) may operate in an infrastructure mode. Infrastructure mode defines a set of communication protocols, one of which is an association protocol for a station to join an access point, another of which provides for an access point which operates continuously to periodically communicate with a station which has the characteristic of being in a power-down (sleep) mode, where the station periodically is activated into a power-up (wake-up) state to receive a Delivery Traffic Indication Map (DTIM) which indicates whether there are packets to be transmitted to the newly-awakened station from the AP. This DTIM power-save protocol conserves a significant amount of station power by only consuming power when the station is in a power-up state (the interval when power is applied to the receiver circuits) shortly prior to the expected arrival of a beacon frame, throughout the beacon frame, and where the power-up state occurs during defined intervals related to periodic beacon intervals. A typical beacon interval is 100 ms. In one example prior art embodiment, the wireless AP transmits a frame known as Delivery Traffic Indication Map (DTIM) and the STA responds indicating how often the station will wake up to check the beacon frame to receive or transmit to the AP. According to the 802.11 power-save protocol for stations, the station is in a power-up state shortly prior to the expected arrival of the AP beacon frame, and the STA stays remains powered on long enough during this throughout the beacon interval to determine whether the traffic indication map (TIM) of the beacon frame indicates there are receive packets destined to the AP, and if so, the station STA remains in a power-up state until the packets are received from the AP, as indicated by the TIM. A prior art station also remains powered up until the arrival of a beacon frame. The TIM includes a header part indicating station assignments and a bitmap part indicating whether a particular station has traffic to receive.
A problem arises in congested networks, where the STA may wake up at the appointed beacon interval, but the beacon frame is delayed in transmission by the transmitting AP because of network congestion (such as from an associated station transmitting, same-channel interference, or a station or other access point on the same WLAN channel is nearby and interfering), in which case the AP waits for a clear channel before transmitting the beacon frame. During either of these disruptions, the STA remains powered up and awaiting reception of the delayed WLAN beacon frame, causing unwanted power consumption. Additionally, each beacon frame must be received and the TIM examined to determine whether the AP has a packet to transmit to the station, including during long intervals where there are no packets to be received from the AP. It is desired to provide a power saving apparatus and method which provides improved power savings in a station operative in congested networks where the station wakes up and the expected beacon is delayed because of network congestion or interferers, and to provide an apparatus and method for reducing receiver power consumption during beacon frame reception.
Another problem for stations infrequently receiving packet traffic is that the power consumed during the time the station is in a wake-up state and waiting for a delayed beacon frame to arrive may be the primary power drain for the station receiver. In this circumstance, the time required to accurately detect the beacon preamble becomes a significant power drain. Accordingly, it is desired to reduce the power consumption of the station during the duration of time waiting for a delayed packet such as a beacon frame.